1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the same.
2. Description of the Related Art
There is conventionally known an image forming apparatus that forms an image on a recording medium such as paper based on image information.
In such image forming apparatus, an image forming section that carries out image formation operates in one of an operating state where image formation is possible and a stopped state where image formation is impossible. For example, an image forming section that carries out image formation according to an electrophotographic technique forms a developer (toner) image on a cylindrical photosensitive drum based on image information, but to do so, in the operating state, it is necessary to rotate the photosensitive drum in advance and to apply a predetermined voltage to the photosensitive drum from a charger section. On the other hand, in the stopped state, it is necessary to stop the rotation of the photosensitive drum and to stop the application of voltage to the photosensitive drum from the charger section. It should be noted here that a predetermined transfer time period is necessary for an operating state transferring process for causing the image forming section in the stopped state to transfer to the operating state and a predetermined transfer time period is also necessary to cause the image forming section in the operating state to transfer to the stopped state.
Some image forming apparatuses also have a function for transferring, when the image forming section is in the stopped state and a predetermined time period has elapsed, for example, to a low-energy mode where power consumption of the entire image forming apparatus is suppressed. In an image forming apparatus equipped with a low-energy mode, when image information obtained by reading an original has been stored in a memory, a problem may occur where the image information stored in the memory is erased when the image forming apparatus transfers to the low-energy mode. For this reason, an image forming apparatus (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H10-149066) has been proposed where the image forming section is prevented from transferring to the stopped state by comparing a remaining number of sheets in a cassette that stores the sheets used for image formation with a number of such sheets required to complete the image formation and reducing the image formation speed (i.e., reducing the number of pages subjected to image formation per unit time) when the number of sheets remaining in the cassette is too low.
There is also the problem that when a plurality of image formation jobs are consecutively executed (here, an “image formation job” refers to a process where image formation is carried out by the image forming section for image information composed of a plurality of pages, for example), the image forming section may transfer from the operating state to the stopped state if preparation for executing an image formation job that follows after completion of an earlier image formation job has not been completed in time. Since the operating state transferring process of the image forming section is executed after the preparation for the succeeding image formation job has been completed (for example, an expansion process that expands image information in page description language format to image information that can be processed by the image forming section has been completed), there is the problem that the following image formation job cannot start immediately. For this reason, an image forming apparatus (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2000-181297) that idles (an operation where a photosensitive drum and the like are rotated and the image forming section is kept in the operating state) without a stopped state transferring process where the image forming section transfers from the operating state to the stopped state being executed after the preceding image formation job is completed is also known.
However, with the image forming apparatus of the above-mentioned Japanese Laid-Open Patent Publication (Kokai) No. H10-149066, there is the problem that if the image formation speed is reduced, that is, the interval for feeding sheets on which images are formed is increased, productivity (the number of pages on which images are formed per unit time) will fall during the time period where the sheet feeding interval is increased.
Also, with the image forming apparatus of the above-mentioned Japanese Laid-Open Patent Publication (Kokai) No. 2000-181297, if the stopped state transferring process is executed in a case where an error that makes image formation impossible has occurred, such as when there are no more sheets for use by the image forming section during execution of an image formation job (that is, there are no more sheets in the cassette storing the sheets in use), it will not be possible to resume the interrupted image formation job until execution of the stopped state transferring process and the operating state transferring process has been completed.